Semiconductive differential photodetector for two dimensional discrimination



7 Y i; June 5, 1962 R K MUELLER 3,038,079

SEMICONDUCTIVE DIFFERENTIAL PHOTODETECTOR FOR TWO DIMENSIONALDISCRIMINATION Filed Jan. 13, 1959 DIFFERENTIAL FIG CELL WW :WM/J':

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ROLF K. MUELLER ATTORNEY United States Patent SEMICONDUCTIVEDIFFERENTIAL PHOTODE- TECTOR FOR TWO DIMENSIONAL DISCRIMI- NATION RolfK. Mueller, St. Paul, Minn., assignor to General Mills, Inc., acorporation of Delaware Filed Jan. 13, 1959, Ser. No. 786,491

6 Claims. (Cl. 250-403) This invention relates generally tophotoelectric systems and pertains more particularly to a system of thischaracter in which a semiconductive member containing a grain bounda isused in the two dimensional discriminatm ing light spot.

The utilization of semiconductive material having a grain boundary forone dimensional discrimination is not new, as it is well known that thetransversal photovoltage changes as a light spot crosses the grainboundary. See, for instance, the article written by G. L. Pearsonappearing in the Physical Review, vol. 76, page 459 (1949) and US.Patent 2,740,901, issued to Robert E. Graham on April 3, 1956. However,the usefulness of semiconductive devices for discrimination, and morespecifically tracking of a light spot, in only one dimension is somewhatlimited, as one can readily understand.

Accordingly, one object of the present invention is to provide a grainboundary photovoltaic cell capable of discrimination in two dimensions.More specifically, it is an aim of the instant invention to employ themore basic ohmic contact scheme heretofore used at either end of asemiconductive member or cell having a grain boundary, but to supplementthese end contacts with additional contacts applied at the grainboundary itself, whereby the movement of a light spot may be accuratelytracked or followed, even though it moves in two directions.

Through the envisaged multiple contact arrangement it is possible tomake use of a lateral photovoltaic effect in addition to the previouslyrealized transvers al photovoltaic effect. The ombined photovoltaicmanifestations so achieved provide atifir'ate indications or signals ofthe digressions or deviations of the light spot from intersectingreference lines. This is because any shifting of such a light spot in adirection away from either of two reference lines will cause a change inthe polarity of the voltage, depending on the direction of spotmovement, and in voltage maguitude, depending on the degree of displaceBrent. In this way, the light spot can be tracked as it travels in anytwo coordinate directions.

Other objects will be in part obvious, and in part pointed out more indetail hereinafter.

The invention accordingly consists in the featurse of construction,combination of elements and arrangement of parts which will beexemplified in the construction hereafter set forth in the scope of theapplication which will be indicated in the appended claims.

In the drawing:

FIG. 1 is a schematic view in perspective of a photoelectric trackingsystem embodying the teachings of the present invention;

FIG. 2 constitutes a curve depicting the transversal photpvoltaicresponse as a function of light spot position for a giv'rfliglitintensity and spot diameter, and

FIG. 3 presents a pair of curves graphically illustrating the respectivelateral photovoltaic responses for two different light spot positionsbut with equal light intensities and spot diameters.

Referring now in detail to FIG. 1, a semiconductive cell denotedgenerally by the reference numeral is schematically pictured. Whileother semiconductive materials, such as silicon, and the III-Vcompounds, may be employed, it will be assumed for the sake ofdiscussion that the cell 10 is a small slab of n-typ e germanium of rec-"ice tangular cross section. Since the cell 10 has been greatly enlargedin FIG. 1 with respect to the rest of the apparatus, it might be well toexplain that exemplarly dimensions for the cell 10 are as follows:length, 6.0 millimeters; width, 2.0 millimeters; and height, 0.5millimeter. Of course, these dimensions are in no way restrictive as awide choice of sizes is possible. The cell 10 has a grain boundary at 12extending perpendicularly to its main axis which renders the celldifferential in character, as will be more fully understood as thedescription progresses. A pair of ohmic contacts 14, 16 are applied atthe end of the cell and a pair of indium contacts 18, 20 are alloyed onopposite sides of the grain boundary 12.

To illustrate a practical application of the invention for trackingpurposes; it will be assumed that the movable target is in the form of asmall light source 22. As denoted by the arrows 24, 26 the target sourceor spot 22 may move in opposite directions in either of two dimen-510115.

The target spot 22 may originate from a number of different sources. Forinstance, it may be a star, a light mounted on an airborne vehicle, aspllggg of illumination carried on a movable machine tool, and so on.

' Light from the target spot 22 is directed through a fixedly locatedfocusing lens 28 onto a tracking mirror 30. From the mirror 30 the lightis reflected onto the differential cell 10 in the form of a minute,concentrated spot 32. More will be said presently concerning the roleplayed by this spot.

Since the purpose of the mirror 30 is to actually track or follow themovement of the source 22, the mirror must be mounted in a way that itcan be moved into various angular positions. To do this the mirror 30,in the illustrative instance, has been mounted for tilting movementabout a horizontal azis provided by an upstanding gimbal 34, the gimbalin turn being rotatable about a vertical axis. The tilting or rotatingmovement of the mirror 30 about a horizontal axis has been indicated bythe arrow 36, whereas the rotation about the vertical axis has beendenoted by the arrow 38.

The rotation of the mirror 30 about a vertical axis is effected by afirst servomechanism 40 at the base of the gimbal 34, and the verticalrotation is accomplished by a servomechanism 42, mounted on the gimbalso as to be movable therewith. The servomechanisms 40, 42 are ofconventional construction and need not be described in detail other thanto say that each is responsive to voltage polarity and preferably tomagnitude, too. Inasmuch as We will be dealing with relatively smallpotential signals, an amplifier 44 is connected in circuit with theservo 40, and an amplifier 46 is similarly connected in circuit with theservo 42. One side of the amplifier 44 is connected to the contact 16and by grounding the contact 14 and the servo 40 any photovoltaic signaldeveloped between the contacts 14, 16'1's impressed on the servo 40 viathe amplifier 44. By the same token, one side of the amplifier 46 isconnected to the contact 20, and by connecting the contact 18 and servo42 any photovoltaic signal developed between the contacts 18, 20 isapplied to the servo 42 by way of the amplifier 46.

To provide a better understanding of the invention, FIG. 2 has beenpresented and shows a typical response curve, designated by the numeral50, involving the photovoltage as a function of the position of thelight spot 32. As a matter of interest, it can be pointed out that theparticular cell 10 from which the curve shown in FIG. 2 has been derivedcontains a 25 tilt boundary. In FIG. 2 the photovoltage has been plottedin millivolts against light spot position in millimeters. Consequentlyit is evident that as the light spot 32 crosses the grain boundary 12there is a sudden reversal in polarity.

It has already been pointed out that there is a sudden reversal inpolarity in voltage as the light spot crosses the grain boundary 12 ofthe cell 10. More specifically, it can be said that the extremes in theresponse curve 50 occur when the perimeter of the light spot. 32 isadjacent to the boundary.

In any event, use is made of the reversal in polarity of the transversalphotovoltage to control the direction in which the servo 40 acts. If novoltage is developed, then the spot 32 is on the grain boundary 12, andno correction of the mirror 30 is needed. On the other hand, if the spot32 has been shifted to, say, the right, a positive voltage signal isgenerated that calls for a rotation of the gimbal 34 in acounter-clockwise direction when viewing the arrow 38. Such a rotationwill re-orient or reposition the mirror 30 so that the spot 32 isreturned to a null position straddling the grain boundary 12. If,instead, the spot 32 has been displaced to the left of the grainboundary 12, as viewed in FIG. 1, then an opposite correction takesplace, for under these circumstances a negative photovoltage will havebeen developed.

Corning now to an explanation of the lateral photovoltaic effectattention is now directed to FIG. 3. In FIG. 3, two separate anddistinct curves 52 and 54 are presented. The curve 52 was derived from alight spot 32 moving along the grain boundary 12, much as it appears tobe doing in FIG. 1. In other words, the light spot which we have labeled32 would be straddling the grain boundary 12 and would be moving, say,from the indium contact 18 toward the opposite indium contact 20. On theother hand, the curve 54 was derived by moving the light spot 32parallel to the boundary 12, and, in the present instance, the' distanceor spacing from the boundary was 0.2 millimeter. Explained somewhatdifferently the light spot 32 in producing the curve 54 could beconsidered to be shifted 0.2 millimeter to the right from the positiondepicted in FIG. 1. After having done so, then the light spot 32 couldbe considered as moving in the same direction as when it was straddlingthe grain boundary to produce the curve 52. This response, contrary tothe transversal effect represented by curve 50, depends on theseparation of the indium contacts 18 and 20 and increases withdecreasing separation. The lateral photoeifect is connected with thep-type inversion layer at the boundary 12.

Thus, while a transversal photovoltaic effect is observed when the lightspot 32 moves between the ohmic contacts 14 and 16, a lateral ofphotovoltaic effect is observed when the light spot 32 moves between theindium contacts 18 and 20. In both instances a change in polarityresults if the light spot is moved through a pair of intersectingreference lines. In the first instance, that is where the transversalphotovoltaic manifestation is involved, the reference line is the grainboundary 12, and with respect to the lateral photovoltaic effect, it isa line of symmetry which is an imaginary line intersecting the grainboundary at right angles and passing through a point midway between theoontacts 18, 20. With further respect to the lateral photovoltaiceffect, it will be seen that the response for any given deviation fromthe symmetry line is highest if the light spot straddles the boundary,such a condition being represented by the curve 52. The maximum responsefor a given distance from the boundary 12 occurs with the light spot 32close to either indium contact 18 or 20.

Consequently, in the latter instance, when the spot 32 has been shiftedto a point, say, above the line of symmetry, a positive photovoltagesignal will have been developed that will require that the mirror 30 betilted in a clockwise direction when viewing arrow 36. The servo 42 doesthis as it receives an amplified signal calling for its operation insuch a direction. The converse is, of course, true when the spot 32 dipsbelow the line of symmetry, for then the polarity reverses, as isevident from the illustrative curves 52, 54, the servo 42 then acting inan opposite direction to raise the spot 32.

From the foregoing information, it can be appreciated that twodimensional discrimination can readily be achieved with a singlesemiconductive cell 10 containing a grain boundary which has beenequipped with additional contacts 18, 20 alloyed thereto at the oppositesides of the cell and at the ends of the grain boundary. Obviously mysystem and method are not limited to tracking as use thereof can be madefor other purposes where two dimensional discrimination is required. Inthis regard, instead of the servomechanisms 40, 42 suitable voltmeters,cathode ray oscilloscope and the like might be substituted.

As many changes could be made in the above construction and manyapparently widely different embodiments of this invention could be madewithout departing from the scope thereof, it is intended that all mattercontained in the above description or shown in the accompanying drawingsshall be interpreted as illustrative and not in a limiting sense.

It is also to be understood that the lauguage used in the followingclaims is intended to cover all of the generic and specific features ofthe invention herein described and all statements of the scope of theinvention which, as a matter of language, might be said to falltherebetween.

What is claimed is:

1. A two dimensional discriminating system of the photoelectric typecomprising a semiconductive cell containing a grain boundary, a firstpair of contacts at the opposite ends of the cell remote from said grainboundary, a second pair of contacts at the opposite sides of the cell atsaid grain boundary, means for impinging light onto said cell in thefor-m of a spot from a movable light source, first means in circuit withsaid first contacts for providing an indication of the transversalphotovoltaic effect produced by said light spot, and second means incircuit with said second contacts for providing an indication of thelateral photovoltaic effect, whereby the relative position of said lightsource may be determined from said photovoltaic effects.

2. A photoelectric system in accordance with claim 1 in which said cellis of germanium, said first contacts form an ohmic connection with saidcell, and said second contacts are of indium and are joined to said cellat the terminations of said boundary.

3. A two dimensional discriminating system of the photoelectric typecomprising a semiconductive cell containing a grain boundary, a firstpair of contacts at the opposite ends of the cell remote from said grainboundary, a second pair of contacts at the opposite sides of the cell atsaid grain boundary, a movable source of light, means for directinglight from said source onto said cell in the form of a spot, first meansin circuit with said first contacts for providing a first signal inaccordance with the transversal photovoltaic efiect produced by saidlight spot, means responsive to said first signal for returning saidlight spot to a reference location in one direction on said cell when ithas shifted due to movement of said source in one dimension, secondmeans in circuit with said second contacts for providing a second signalin accordance with the lateral photovoltaic effect produced by saidlight spot, and means responsive to said second signal for returningsaid light spot to a second reference location in a second direction onsaid cell when it has shifted due to movement of said source in a seconddimension.

4. A photoelectric system in accordance with claim 3 in which the grainboundary constitutes the first reference location and the line ofsymmetry between said second contacts constitutes the second referencelocation.

5. A two dimensional discriminating system of the photoelectric typecomprising a semiconductive cell containing a grain boundary, a firstpair of contacts at the opposite ends of the cell remote from said grainboundary, a second pair of contacts at the opposite sides of the cell atsaid grain boundary, a movable source of light, a mirror for directinglight from said source onto said cell in the form of a spot, firstmotive means for moving said mirror in a direction to return said spotto said grain boundary, second motive means for moving said mirror in adirection to return said spot to a position on the line of symmetrybetween said second contacts, first means in circuit with said firstcontacts for energizing said first motive means in accordance with thetransversal photovoltaic effect produced by said light spot when shiftedaway from said grain boundary to effect its return to said boundary, andsecond means in circuit with said second contacts for energizing saidsecond motive means in accordance with the lateral photovoltaic etfectproduced by said light spot when shifted away from said line of symmetryto effect its return to said line.

6. A dimensional discriminating photodetector comprising a lightsensitive semiconductive cell containing a single grain boundaryseparating regions of like semiconductive material for generatingsignals indicative of the location of light on said cell, ohmic contactsconnected to said separate regions and remote from said boundary,boundary contacts joining said regions at the terminations of saidboundary, means for directing a concentrated light onto said cell from amovable light source, and means connected to said ohmic and boundarycontacts and responsive to said signals for orienting said directingmeans in accordance with said signals.

References Cited in the file of this patent UNITED STATES' PATENTSKircher 76/5 June 9, 1953

